EN 12326 - Petrography, Mineralogy & Fabric

A petrographic investigation has to be carried out for the correct determination of the rock type. In other words whether the slate is really a slate and thus belongs within the range of the norm. In addition, an identification can be done with the petrographic investigation. Especially the petrographic/mineralogical investigation is important to determine the mineral components and their possible influence to the weathering behaviour (colour, strength). The EN-12326 states a 800x magnification. Earlier investigations showed that this magnification is rather unusable because neither the determination of the morphology nor of the thickness of the mica layer is possible. A 100x to 250x magnification should be absolutely sufficient and it turned out that even the 250x magnification is to large for the quantification by means of the image analysis.

Investigations

Macroscopic Investigations

This investigation is carried out on a hand specimen and the following criteria has to be incorporated in the test report:

• sedimentary bedding with the differentiation of:
  - slates where bedding and the fracture cleavage encompass an angle (= fanning fracture cleavage);
  - slates where bedding and the fracture cleavage run parallel to each other;

• open and sealed cracks;

• transverse and conjugate joints;

• calcite or other carbonates in the form of veins or thin layers;

• coaly material;

• occurrence of pyrite, marcasite, pyrrhotine (all iron sulfides) or other metallic shining minerals

• if carbonates occurs whether the mica layers surround this aggregates;

Microscopic Investigations

This investigation are carried out by means of a polarisatons microscope and the following criteria has to be incorporated in the test report:

• in case of slates, containing carbonate (calcite, ankerite) or ore minerals (pyrite, ilmenite), it has to be described whether they are homogeneously distributed within the ground mass or as single lenses or in form of veins or serrated nodes;

• occurrence of sealed joints, traces of a former bedding as well as a former cleavage plane;

the sealing and distribution of the fabric and structure of the mica layers has to be described as follows:

• structural type of the mica layers

• spacing: number of mica layers per mm

• average thickness of 10 mica layers

• value of amount

Because of the high importance of the mica layers for the mechanical- and finishing properties of a slate the investigations should focus on them.

Remark about the classification within the norm:
A first classification was established by Hirschwald in 1908 and further developed by Wagner in 1989 which corresponds to the EN-classification of the cleavage plane. The problem of this classification is that there no real differences between cleavage types exist and thus this division is rather arbitrary: differences, for example, between 3C and 4C or between 3B and 3C cannot really reconstruct. Another problem is the impossibility to derive any properties of a slate. A main separation between continuous and spaced foliation does not exist. Because of the aforementioned, neither a description nor a real differentiation is possible with this classification and makes it highly impracticable. Further information about the fabric of a roofing slate can be found here.

Mineralogical Composition

The mineralogical composition should be determined by X-ray analysis. Because of the very fine grained sedimentary pattern or ground mass, respectively, estimations within the microscopic picture are very faulty. In addition, the results are not comparable with those of the X-ray analysis.